If your service is an ADSL one, first check to see what the line sync figures are, then look at the downstream attenuation figure.
I'm able to log downstream attenuation from my DSL modem with collectd[1]. Sync rates could likewise be logged. This is straightforward as long as the modem provides this data over SNMP. My modem is a Netopia 3341.
[1] http://collectd.org/